You can toggle on and off the various icons: city names, camps, baths, caves, grindstones, taverns, millers, herbalists, etc. To add your own local marker, just click on the map, then click Add Marker. Choose the icon you want (which include a few general, all-purpose icons like arrows, stars, and exclamation marks) and label it. It's a good way to keep track of interesting or useful locations you come across that might not appear on your in-game map (for instance, places you spot certain herbs for alchemy, or a shop selling something you can't afford but want to buy later, or an area you plan to explore further for treasure).

Currently, only the area around Skalitz has been marked up officially on the map, though I expect it won't be long before the rest of the map begins to fill up. Rogerhnn's post lists some features planned, like the ability to share your version of the map and all of your custom markers with others. (I also hope there will be a way to save all my custom map icons somewhere other than just my browser history, because I don't trust my browser that much). The post also details how you can help fill out the map by submitting markers either through Discord or Reddit.